Tauopathies are a group of neurodegenerative disorders in which filamentous tau aggregates are a hallmark pathological lesion. The recent discovery of tau mutations in FTDP-17 demonstrates that tau dysfunction can directly cause neurodegeneration. However, the mechanism underlying the formation of neurofibrillary tangles (NFT) is unclear. Work from our laboratory demonstrates that transglutaminasecatalyzed cross-linking of tau is present in NFT in two tauopathies. Transglutaminase is a calciumdependant enzyme that covalently cross-links proteins rendering them insoluble, similar to the properties of NFT. It is not known what causes the increased cross-linking of tau in tauopathies. Preliminary data from our laboratory suggest that oxidative damage and elevation of intracellular calcium produce elevated transglutaminase-catalyzed cross-links in tau. We hypothesize that oxidative stress and disruption of calcium homeostasis may contribute to the transglutaminase-catalyzed cross-linking of tau and neurofibrillary pathology of tauopathies. Our first aim is to demonstrate that transglutaminase-catalyzed cross-links are present in tau from FTDP-17 patients and mice expressing VFDP-17 associated tau mutations. The second and third aim will utilize a cell culture model to assess the effects of oxidative stress and elevated intracellular calcium on the transglutaminse-catalyzed cross-linking of mutated tau. The proposed studies will help elucidate mechanism of NFT tangle formation, therefore providing potential avenues for therapeutic intervention in a broad range of tauopathies.